The discovery of the 401 Hz accreting millisecond pulsar IGR J17498-2921 in a 3.8 h orbit

Abstract

We report on the detection of a 400.99018734(1) Hz coherent signal in the Rossi X-ray Timing Explorer (RXTE) light curves of the recently discovered X-ray transient, IGR J17498 - 2921. By analysing the frequency modulation caused by the orbital motion observed between August 13 and September 8, 2011, we derive an orbital solution for the binary system with a period of 3.8432275(3) h. The measured mass function, f(M₂,M₁,i) = 0.00203807(8) M_⊙, allows to set a lower limit of 0.17 M_⊙ on the mass of the companion star, while an upper limit of 0.48 M_⊙ is set by imposing that the companion star does not overfill its Roche lobe. We observe a marginally significant evolution of the signal frequency at an average rate of - (6.3 ± 1.9) × 10^-14 Hz s^-1. The low statistical significance of this measurement and the possible presence of timing noise hampers a firm detection of any evolution of the neutron star spin. We also present an analysis of the spectral properties of IGR J17498 - 2921 based on the observations performed by the Swift-X-ray Telescope and the RXTE-Proportional Counter Array between August 12 and September 22, 2011. During most of the outburst, the spectra are modeled by a power-law with an index Γ ≈ 1.7 - 2, while values of ≈ 3 are observed as the source fades into quiescence.